This story, following on from Deimos, takes a darker turn as peoples' lives come into the balance, in many different scenarios. The Doctor is forced to make judgment calls that don't necessarily sit well with those he is having to work with. Without putting in any spoilers, you can tell that this story is one that is continuing down the path of a set up to a big finale of the season.
What I like about this story is that it again seems to be quite mindful of the "classic" Doctor Who stories - the Doctor is involved in the action, but is also torn with moral dilemmas, finds himself having to justify his motivations, and is caught up in the Web of Time. The old enemies (without being too specific) that again cross his path are welcome returns. Where the story leads from here will be the next step of the journey for us all - the suspense is building in these stories that are obviously leading up to a big confrontation somewhere along the way. Can't wait!